High-frequency-wave signaling and communicating system



Feb. 9 1926.

E. H. SMYTHE ET AL HIGH FREQUENCY WAVE SIGNALING AND COMMUNICATING SYSTEM Filed May 14, 1921 5 SheeIs-Sheet 1 m. m V m Af/y I Feb. 9 1926.

v E. H. SMYTHE ET AL HIGH FREQUENCY WAVE SIGNALING AND COMMUNICATING SYSTEM v Myenfars: [dud/7 /7. Smyf/re, jamae/fi. /4/////'4/775 Jr by M Af/y 5 Sheets-Sheet 2 Filed May 14, 1921 Feb. 9 1926. 1,572,756

E. H. SMYTHE ET AL HIGH FREQUENCY WAVE SIGNALING AND COMMUNICATING SYSTEM Filed May 14, 1921 5 Sheecls-Sh eet 5 by M Affj/ Feb. 9 1926. 1,572,756 E. H. SMYTHE ET AL HIGH FREQUENCY WAVE SIGNALING AND COMMUNICATING SYSTEM Filed May 14, 1921 5 Shets-Sheet 4 Feb.- 9 1926.

E. H. SMYTHE ET AL HIGH, FREQUENCY WAVE SIGNALING AND COMMUNICATING SYSTEM.

Filed May 14. 1921 5 Sheet -Sheet 5 m x N 295cm Patented Feb. 9, 1926.

, UNITED STATES I 1,572,756 PATENT OFFICE. v

EDWIN H. SMYTHE, OF EVANSTON, ILLINOIS, AND SAMUEL B. WILLIAMS, JIL, OF

BROOKLYN, NEW YORK, ASSIGNORS TO WESTERN ELECTRIC COMPANY, INCOR- PORATED, OF NEW YORK, N. Y., A CORPORATION-OF NEW YORK.

HIGH-FREQUENCY-WAVE SIGNALING AND COMMUNICATING SYSTEM.

Application filed May 14, 1921. Serial No. 469,898.

. To all whom it may concern:

Be it known that we, EDWIN H. SMYTHE and SAMUEL B, IVIL'LIAMS, Jr., citizens of the United States, residing at Evanston, in the county of Cook and State of Illinois, and at Brooklyn, in the county of Kings and State of New York, respectively, have invented certain new and useful Improvements in High-Frequency-lVave Signaling and Communicating Systems, of which the following is a full, clear, concise, and exact de-- scription. I

This invention relates to an improved communicating system, and more particularly to a communicating system of the type in which high frequency waves or oscillations, either free or guided, are employed as the medium for conveying between the different stations of the system the ener y for controlling the operation of the electric signaling, controlling and communicating apparatus.

Intercommunication between the stations of a high frequency system is effected through the medium of what are commonly known as high frequency channels, each channel consisting of apparatus for transmitting and receiving a wave of a certain frequency as distinguished from the apparatus used in transmitting and receiving a wave of a different frequency; or, in the case of a channel for carrying on conversation or two-way communication between two stations, the channel may comprise the apparatus for transmitting and receiving waves of two different frequencies, one of the waves being used for transmitting in one direction and the other wave for transmitting in the oth r direction. The characteristics of these channels are such that each is capable of transmitting and receiving only its own particular frequency or set of frequencies, so that simultaneous use of the channels is possible without interference between them. A comniunicating system employing channels of this type is shown in U. S. Patent No.

1.499.081 issued to E. H. Smythe and S. B. \Villiams, June 24, 1924. The number of non-interfering channels that may be provided in this class of systems is, however, limited; audit is the particular object of the present invention so to organize a high frequency system that a relatively small number of non-interfering channels may be used with the highest etliciency in affording controlling and communicating facilities between a number of stations and so that the maximum of flexibility in thematter of the trunking or communicating facilities of the system may be attained.

The present invention contemplates a high frequency system in which intercommunication between the various stations of the system is effected through the medium of a number of such non-interfering high frequency channels, each of which may be available to all of the stations. Each of the stations of the system is so organized that not only may it establish connection with any other station of the system by way of any of the high frequency channels, but it may also utilize simultaneously all of the channels that are not at the time in use for the interconnection of other stations of the system. Furthermore, the system of the invention is so organized that each station has means for keeping at all times in operative relation with one of the channels and available for incoming or outgoing calls-one or another of a number of trunks or channel extensions, so that the initiation of a call either from or to the station finds an outlet or inlet for the call at once available; and when one of the trunks or extensions is taken for use, another is automatically preselected and placed in operative relation with the channel for the use of the next call. This calling channel is therefore kept constantly in operative relation with all of the stations of the system, and with an idle or available trunk at each of the stations.

In order to simplify the system, effect economy in the apparatus required, and realize certain advantageous features of operation, the system of the invention is further so organized that only such of the elements of each trunk as are directly involved in the separate connection established over the trunk are individual to the trunk, the other elements that are involved in the receipt of a call, the selection of the particular station called, the selection of a communicating channel and the control of the trunk in the setting up of the connection being common to all of the trunks, and being automatically associated with or attached to each trunk when their co-operation is r uired, and thereafter automatically detache and associated with the next trunk tobe used.

In addition the system of the invention in the preferred form in which it is disclosed, embodies a number of other features incidental and auxiliary to those mentioned above. The manner in which the above mentioned features, and the features auxiliary or incidental thereto, are realized will v be explained in the following description of the preferred form of the invention, and the structures and combinations through which the objects of the invention are attained will be more particularly defined in the appended claims.

The preferred embodiment of the invention is illustrated in the accompanying drawings, in which Figs. 1 to 4, inclusive, show diagrammatically as much of the circuit organization as is necessary to enable the invention and the preferred manner of practicing it to be understood, and in which Fig. 5 is a schematic outline of the general idea or plan of'the system. In assembling Figs. 1 to 4 of the drawing to show the circuit organization, they are arranged as indicated in Fig. 6.

Referring first to Fig. 5, this illustrates schematically two of the plurality of stations which may be assumed to be adapted for intercommunication through the medium of the high frequency communicating system of the present invention. These stations are indicated in the present instance as radio stations, that is, stations between which interconnection and intercommunication are established through the medium of unguided high frequency waves transmitted through space. In the specification that follows the high fre uency stations may occasionally be referre to as radio stations; but it is to be understood that the use of this term is not intended to limit the invention The two stations shown in Fig. 5 are indicated station No. 1 and station No. 2, respectively. Each of the stations 1s indicated as having, three trunks, trunk 1, trunk 2 and trunk 3. Each of these trunks is represented as terminating at one end in spring jacks located respectively at position 1 and position 2 of the switchboard, and at the other end in tuning or channel selecting switches S, S and S, co-operating with radio or high frequency communicating apparatus, which is indicated in the present instance as merely a single antenna designated TA and BA with the associated calling channel BX and communicating channels B B and B, but which in \the present embodiment of theinvention stands for separate transmitting and receiving antennae systems. Each station is assumed to have a number of associated local lines with which the three trunks of the station may be connected through the medium of the connecting circuits 37 shown at the two switchboard positions.

The interconnection of the trunks of one station with the trunks of another station is through the medium of three pairs of high frequency communicating waves and a high frequency calling wave, indicated in the present instance by the broken lines designated communicating waves and calling wave, respectively. The ability of each trunk to be brought into operative relation alternatively with the calling wave or any of the pairs of communicating waves is in dicated by the antennas TA and RA being connectedthrough. the channels BX, B, B and B with the stationary terminals of the associated tuning switch. normal position each switch brings its associated trunk into operative relation with the calling wave through the medium of the calling channel BX and the antennae TA and RA. In each of its other three positions, each switch establishes operative relation with one of the pairs of communicating waves through the medium of the corresponding channel B B or B.

The three trunks of each station are represented as being adapted to be brought into operative relation with the apparatus. designated common sender and register circuit through the medium of relays 11, one of which is associated with each of the three trunks. Each trunk is also indicated as having associated with it a relay 16 that is adapted when operated to complete operative connection between the trunk and its In its first or particular antennae TA and RA through the medium of the associated tuning switch and the calling channel BX. The schematic representation is such as to indicate that when the tuning switch is in its normal position to tune the associated antenna to the calling wave, the operative connection between the trunk and the antenna is under the control of the relay 16; while the movement of the switch to tune the antenna to any one of the pairs of communicating Waves establishes a direct operative relation between the antenna and the trunk at the lower contacts of the switch. As represented, therefore, when the antennae associated with any'of the trunks are-tuned to one of the pans of communicating waves,

.their operative condition is independent of the connection by way of the contacts of the associated relay 16.

-Associated with the three trunks at each station is common apparatus for determin ing which of the trunks shall next be used for incoming or outgoing calls, and which is designated preselecting apparatus. This apparatus is shown in the schematic representation of Fig. 5 asthree switch arms, each provided with a contact point for each trunk, the connections extending from the switch arms and their contact points being such that in whichever position the switch is standing the trunk corresponding to that position has control over its multi-contact relay 11, while the associated trunk relay 16 is actuated to make operative the antennae TA and RA of that trunk. Therefore, for each of the three positions that the preselecting apparatus may occupy, a corresponding one of the trunks has its antennae mad-e responsive and operative by way of the calling channel, and the common sender and register circuit placed in readiness to become attached to that trunk when the trunk is energized either through one of its conductive terminals or through its radio or high frequency terminals. \Vhen the common sender and register circuit is at-v tached to the particular trunk determined by the position of the preselecting apparatus. it is adapted to control and be controlled by the apparatus of the associated trunk, and to tune the antenna or high frequency terminal of the trunk, through the medium of the corresponding tuning switch to the par ticular pair of communicating waves that is selected for use.

As has been stated, Fig. 5 is merely an attempt to illustrate schematically the general idea or plan of arrangement of the system, and does not pretend to show anything more than this. Furthermore, the invention contemplates such multiplication of the parts or elements indicated in Fig. 5 as may be desirable in practice. There may be as many stations employed as may be desired.

There may also be any desired number of communicating waves or channels for taking care of the traffic between the various high frequency stations of the system; and each station may have as many trunks as there are communicating channels, so that if necessary any one of the stations may utilize all of the channels simultaneously for communication with one or more of the other stations.

Referring now to the preferred circuit organization as illustrated in Figs. 1 to t inelusive of the drawing, only as much of the wiring 'of a single station is shown as is necessary to enable the invention to be understood, it being possible for one skilled in the art, by mere extension and duplication of the wiring and apparatus illustrated, to organize a system of such capacity as may be desired. As illustrated in the circuit diagram of Figs. 1 to 4 inclusive, the toll station is provided with a plurality of trunks or high frequency channel extensions. The wiring of one of these trunks and the associated high frequency apparatus is shown in detail in Figs. 1,3 and 4; and two other trunks are indicated by additional relays 10 and 11 and spring jacks 30 and 33 such as those of the trunk that is completely shown. The Wiring of the two trunks thus suggested in the diagram may be assumed to be similar to the wiring of the trunk that is shown. Each of the trunks is associated with high frequency terminal apparatus consisting in the present instance of the transmitting antenna TA and the receiving antenna BA with the various elements that constitute and control the associated high frequency channels; and also with certain conductive terminals consisting in the present instance of the answering springjack 30 and two multiple springjacks 33. The multiple springjacks 33 in accordance with the usual practice are located upon different sections of the toll board. Connection with the conductive terminals of the high frequency trunk is made in the present instance through the medium of connecting cord circuits, of which one, designated 37, is shown and another indicated, it being assumed that there are provided as many such connecting circuits as traffic conditions require. Each of the connecting cord circuits 37 is illustrated as being provided with a connecting plug 36 adapted for insertion in the spring jacks 30 and 33, a calling key 5, a calling dial 39, a supervisory lamp 6, a supervisory relay 38, an operators telephone outfit 7, and'a listening key 8. The other end of the connecting circuit, which is not shown in de tail, is indicated as terminating in a plug 36; but it may be assumed to be adapted. in any one of the ways well known in the art, for either manual or automatic connection with such other circuits as may require extension to the high frequency trunk. The answering springjack of the trunk has associated with it a line lamp 29, the illumination of which indicates an incoming call over the trunk; and each of the springjacks 30 and 33 has associated with it an assigning lamp 31 for indicating to the operators the availability of the trunk for an outgoing call, and a busy lamp 32 for indicating the condition of use of the trunk.

Each of the stations has a common sender and register circuit which is common to all or a pluralitv of the trunks of that station, and to the circuits of which any one of the trunks may be temporarily extended through the medium of the contacts of a multi-contact relay 11 individual to the trunk. Which of the trunks of the station shall next 'be used and brought into operative relation with the common sender and register circult of Fig. 3, is determined by a control exercised through the medium of a series starting or preselecting circuit including the'conductors 12, 15 and 14 that extend between the contacts of the relays 10 of the various trunks; and a relay 40 common to all of.

the trunks restores the normal condition of the preselecting circuit when all of the trunks in the series have been once used.

The common sender and register circuit of the station, as illustrated in Fig. 2, has associated with it receiving register switch apparatus and transmitting register switchapparatus employed when the trunk to which the common circuit is attached, is being used for calling another radio station, and additional station selecting switch apparatus and receiving register switch apparatus employed when the associated station is being called by some other radio station. The calling receiving register switch apparatus consists of a station receiving register switch A and a carrier wave receiving register switch B, the calling transmitting register' switch apparatus consists of a station transmitting register switch 0 and a carrier wave transmitting register switch E. Each of these transmitting register switches has associated with it a change-over switch, D and F respectively for interposing an interval between the operation of the associated switch and the operation of the next switch or apparatus in the operating sequence. The receiving register apparatus that-comes into use when the associated radio stat-ion is being called by another radio station comprises a station selectingswitch H and a carrier wave receiving register switch K. The common sender and register circuit of each station also has a tuning switch G that operates when the station is the calling station to tune the transmitting antenna and receiving antenna systems of the'associated trunk to the pair of carrier waves registered by the carrier wave register switch B; and which operates when the station is the called station to tune the transmitting and receiving antennae systems to the pair of carrier waves indicated by the carrier wave receiving register switch K. All of the switches referred to above are illustrated, in the present instance, as single motion step-by-step switches, the wipers or switch arms of which are advanced step by step by means of an associated stepping magnet, the return of the switch arms to normal being brought about. by other energizations of the associated stepping magnet that cause the arms to continue their movementover their rows of contact points until they again rest in their normal position. Such step by step switches are well known in the art and need not be particularly described.

Associated with the ,trunks and with the common sender and register circuit in the form in which they are herein disclosed are also a number of plunger relays, as 21, 23, 55, 59, 64, 65, 72 and 76, which are employed to introduce relatively large time intervals into the operation of certain parts of the organization. The plungers of these relays are retarded on their inward movement, and are permitted to move freely on their return I movement. They may be of the typedisclosed in the application of Samuel B. Williams, J r., and Walter P. Albert, Ser. No. 14.6671, filed Feb. 5th, 1917. Two of these slow acting plunger relays or timers 64 and 65 are used, in connection with the associated relays 62 and 63, to constitute an interrupter for producing a slow ser1es of im ulses for controlling the apparatus 'employe in sending out the radio or high frequency signals.

The radio terminals associated with each trunk may consist of two antennae TA and RA. These antennae may be of any desired form, and should preferably be located at a sufiicient distance apart to insure the selectivity of the 'receiving. antenna with respect to the energy in the wave radiated by the transmitting antenna; or, if it is desired to locate the antennae closer together, power limiting devices may be used; or the irresponsiveness of the receiving antenna system to the energy it receives from the transmitting antenna may be attained by any of the several methods that are available in the art.

The medium that is employed in the present instance for effecting radio or high frequency communication between the transmitting antenna TA and the receiving antenna, such as RA, of the distant radio station. is a high frequency carrier wave, the oscillatory current for roducing which is generated in'the tuned high frequency oscillator 171. The oscillator circuit may be of well known type, comprising a vacuum tube or thermionic oscillator with an oscillation circuit including inductively related inductances in the input and output circuits of the tube. The filament current for this oscillator and for the energization of the other oscillators and for the modulator and power amplifier of the transmitting antenna system is supplied by the filament current gcnerator 148, and the plate current for these devices is supplied by the plate current generator 147. These generators are driven by the motor 146, the current supply for oper- -ating which iscontrolled by the relays 199 lator 160 and the other being generated byintermediate frequency oscillatory currents generated by theoscillator 160 are modu ated by the voice currents transmitted from the telephone transmitter of the associated local station. The voice currents from the associated local station pass over the talkin conductors 97 and 98 of the trunk an through the four equal windings of the twoway repeating coil 149 into the balancing network 150. The junction points of the two serially included repeating coil'windings in each side of the line circuit are connected with the input circuit of a thermionic amplifier 151, and the output circuit of the amplifier is connected by way of the inductive couplings 152 and 154 with the input circuit of the modulator 155.

The operation of the oscillating andmodulating circuits herein disclosed, is briefly as follows: The oscillator 160 and modulator 155 are supplied with plate current through the choke coil 161 which has sufficiently large inductance to prevent the passage therethrough ofcurrent fluctuations of frequencies as high as those used in transmission of speech. The plate current supply circuit for the oscillator 160 contains also a choke coil 159 which transmits voice frequencies but has sufiicient inductance to prevent the passage of current fluctuations corresponding to the frequency of oscillator 160. Since the current supplied to the tubes 155 and 160 together in parallel is maintained constant whereas the current through the tube 155 alone is controlled by the impressed voice current, the current through the oscillator 160 is made to vary in a complemental manner, and therefore the amplitude of the wave generated by the oscillator 160 varies in accordance with voice currents received over the line conductors. The frequency of oscillator 160 is controlled by the condenser 157, while the condenser 156, which is too large to have an appreciable effect on the period of the oscillator, serves to prevent short-circuiting of the current supply.

The oscillators 160, 168 and 171 are all connected to and feed into the input circuit of the modulating tube 180. Condenser 178 affords a high frequency by-pass for the radio frequency current from oscillator 171 The output coils of oscillators 160 and 168 are in series. However, the coupling of the oscillatorcircuits to the common work circuit may be made sufficiently loose to prevent reaction of one oscillator upon another and to prevent the constants of the work circuit from affecting the period of the oscillator. I

The action of the modulator 180 is somewhat difl'erent from thatof 155 since all of the waves are impressed upon the grid circuit and the modulating'action depends upon the variation of the tube impedance to the passage of the radio frequency current under control of the currents impressed from oscillators 160 and 168. The result is that the radio frequency wave is modulated by each of the waves from the intermediate frequency oscillators, each of which is in turn modulated or controlled by currents or waves of lower frequencies. The plate current circuit of the oscillator 160 and modulator 155 is controlled by contacts of a relay 215, so that the intermediate frequency generated by the oscillator 160 may be applied to orwithdrawn from the carrier wave to produce signaling impulses. The operation 'of the other intermediate frequency oscillator 1681s controlled at contacts of the relay 216, and the intermediate frequency that it produces is applied to the carrier wave for the purpose of guarding against the production by any other station of the particular frequency that is being transmitted by the oscillator 171. The intermediate frequency produced by the oscillator 168, therefore maybe referred to as the guardin intermediatefrequency and that produce by the oscillator 160 as the signallng and talking intermediate frequency. The output circuit of the modulator 180 is inductively coupled with the tuned input circuit of a power amplifier 209 of the thermionic type; and the output circuit of the amplifier is inductively coupled with the tuned transmitting antenna TA of the system. The tuning of the amplifier input circuit and of the transmitting antenna TA, as well as of the oscillating circuit of the carrier wave oscillator 171, is accomplished through the medium of a plurality of branches, each under the control of a relay: In the embodiment of the invention here disclosed, there are seven tuning branches associated with the high frequency oscillator, seven associated with the power amplifier, and seven associated with the transmitting antenna. The seven tuning branches associated with the high frequency oscillator include adjustable condensers 174, 175, 176, 177, 275, 276 and 277, and these branches are respectively controlled by relays 181, 182, 183, 184, 282, 283 and 284.

The timing branches associated with the there are seven energizing'circuits, and each of these energizin clrcuits'is connected with the-windings of t ree of the controlling relays, one from each of the three tuning roups. The seven energizing pathsfor conuctors are designated 272, 94, 95, 96, 162, 163 and 164, the first of these conductors being connected with the windings of the first relay in each of the three groups in the order in which these relays were mentioned, the second conductor with the winding of the second relay in each group in the order mentioned, and so on. Thus when any one of the energizing paths is closed, the corresponding relay in each of the three tuning groups is o erated, and a corresponding tuning branc is closed.

The capacities and inductances included in the seven branches of each of the three tuning groups are so adjusted that the cloin each of the three groups sure of any one of the seven energizing paths, with the consequent closure of the corresponding tuning branch in each of. the three groups, causes the production and application to the transmitting antenna of an oscillation having a frequency differing from the frequency produced when any'other one of the seven energizing paths is closed. In the present instance, the frequency of the carr1er wave oscillations thus applied to the transmitting antenna may conveniently have the following ratios: 100, 150, 225, 275, 350,

425 and 475, these frequencies being applied by the energization of conductors 272, 94, 95, 96, 162, 163 and 164, respectively.

In the radio receiving part of the system,

the receiving antenna RA of each station is connected with the tuned input circuit of the thermionic detector 230 by means of the inductive coupling 218, 219. The output circuit of the detector includes serially the two.

' the detector 255 is inductively coupled with the input circuit of the amplifier 259. The output circuit of the am lifierhas two branches controlled in switc ing contacts of the relay 271. The normally closed branch is connected by way of the inductive coupling 263 and conductors 102 and 103 with two windin of the two-way repeating coil 149. The inductive relation of these two windings with the other windings of the reare re eated into the trunk talking conductors 9 and 98 connected with the other windings of the repeating coil, but are not repeated into the branch connection of the repeating coil with the transmitting amplifier 151. As a result of this arrangement, which is well known in the art, the voice current modulations that reach the called stat-ion through its receiving antenna are prevented from reacting upon the radio transmitting system of I that station and causing similar voice current modulations of the oscillatory currents passing to its transmitting antenna. The other branch of the amplifier output circuit, closed when relay 271 is operated, is connected through the inductive couplin 264 with the rectifier 269, and the circuit 0 the rectifier includes the winding of relay 268. The relay 268 is adapted to respond to the signaling and talking intermediate frequency of the carrier wave, and to act through the medium of conductor 35 to govern the operation of certain switching mechanism of the asso ciated trunk.

' The filter 235, which selectsthe guarding intermediate frequency of the carrier wave, is connected with the input circuit of the thermionic detector 238, and the output circuit of this detector is inductively coupled with the input circuit of the amplifier 245, the output circuit of which in turn is inductively coupled with the circuit of the rectifier 251. The circuit: of the rectifier includes the winding of relay 252, which relay is thus adapted to respond to the guarding intermediate frequency of the carrier wave and to act through the medium of conductor 100 to govern the operation of switching mechanism of the associated trunk.

The tuning of the receiving antenna system of the radio trunk is effected, in the particular embodiment of the invention lllustrated, by means of seven branches including the adjustable condensers 231, 232, 233, 234, 332, 333 and 334, respectively. These branches are under the control of relays 221, 222, 223, 224, 322, 323 and 324, respectively. The tuning branch containing the condenser 231 and controlled by relay 221 is normally closed, and the other branches are normally open. The winding of relay 221 is included in a common portion of an energizing circuit having the six branches 42, 43, 44, 165, 166 and 167., and

the windings of the other six tuning control relays are included one in each of these six branches. Therefore, when any one of the six branches is closed to operate the included relay and close the corresponding tuning branch, the relay 221 is operated to open the normally closed tuning branch that it controls.

" lays 200 and 199. The relay 200 acts to apthe medium of the relay 215.

. ply the carrier wave and the guarding intermediate frequency when flfi first of the seven tuning branches of the transmitting system is closed, and the relay 199 acts to apply the carrier wave and both of its intermediate frequencies when any of the other six tuning branches is closed. The application of the guarding intermediate frequency to the carrier wave is controlled through the medium of the relay 216, and the application of the signaling and talking intermediate frequency is controlled through Two relays, 242 and 2-43, respectively, control the energization of the signaling or talking and the guarding intermediate frequency branches of the receiving antenna system; and the relay 241, so connected as to operate when either of the relays 242 and 243 operates, controls the energization of the main or carrier-wave-responsive portion of the receiving antenna system. The relays 242 and 243 are so interrelated with the relays 215 and 216 that the application of either the signaling and talking or the guarding intermediate frequency to the calling carrier wave determined by the first or calling tuning branch of the transmitting antenna system, prevents the energization of the corresponding intermediate frequency branch of the receiving antenna system. WVhen the relay 221 acts to break the normally closed tunlng branch of the receiving antenna system upon the closure of one of the other tuning branches, it makes the relays 242 and 243 independent of control by the relays 215 and 216.

,As has been stated, the first tuning branch of the receiving antenna system, corresponding to the calling carrier wave, is normally closed, and the first tuning branch of the transmitting antenna system is closed by the completion of a circuit by way of conductor 272 upon the taking of the trunk for use, and at certain other times in the operation of the system, as will subsequently be described. The other six tuning branches of the receiving antenna system and the other six branches of the transmitting antenna system are selectively energized under the control of the tuning selector switch G. lVhen the radio station is calling, one or another of the transmitting system tuning branches 94, 95 and 96, and one or another of the receiving system tuning branches 42, 43 and 44 are energized, depending upon whether the switch G is moved to its first, second or third position' from normal. \Vhen the radio station is being called, one or another of the transmitting system tuning branches 162, 163 and 164, and one or another of the receiving system tuning branches 165, 166 and 167 are energized, depending upon the position to which the switch G has been moved. The arrangement is such that when the station is calling and the switch G has been moved to its first position from normal, the transmitting system is, tuned, in the present insta cc, to ,a wave length of 150 meters and the receiving, system is tuned to a wave length of 225 meters; while at the same time a corresponding movement of the tuning selector switch G at the called stationito its first position from normal causes the transmitting system of the called station to be tuned to a wave length of 225 meters and the receiving system at that station to be tuned to a wave length of 150 meters. Thus, it will be seen that in this case the receiving system of the called station is tuned to the carrier wave length of 150 meters that the calling station is transmitting, and that the transmitting system of the called station is tuned to a carrier wave length of 225 meters that the receiving system of the calling station is tuned to receive. In like manner when the switches G at the calling and called stations occupy corresponding positions, the trans-- mitting system of each station is tuned to send out a different carrier wave length to which the receiving system of the other station is tuned to respond. This arrangement may be tabulated as follows:

Calling station Called station Switch G 1 Transmit- Transmit- Receiving Receiving g g i wave length fg g z wave length Meters M cters Meters Meters Position 1 150 225 225 150 Position 2 275 350 350 275 Position 3 425 475 475 425 -waiting or preselected trunks of all of the radio stations are normally tuned may be, for instance, 100 meters and the transmitting systems of the preselected trunks of all of'the radio stations are adapted normally to send out this wave. The carrier wave lengths that have been referred to above are merely typical, as any desired combination of wave lengths may be used, provided only that the frequencie'sselected are non-interfering. I

As has heretofore been stated, the apparatus which is used to transmit and receive by means of any one high frequency wave or. air of waves, asdistinguished from that whlch is used onlywhen a wave or pair of waves, of a different frequency-is employed, is commonly known as a high frequency channel. It will be seen that each terminal apparatus .is capable of transmitting or receiving throug any one of a number of channels and that'one such channel may be cies, one of which is modulated to carry the speech frequencies and also is applied and withdrawn to convey the signaling and switch controlling impulses, and the other of which is modulated or controlled to guard against simultaneous use by some other station of the carrier wave to which it is aplied. These intermediate frequencies may iiave any desired value above the audio frequency and below the carrier wave frequency range. For example, the talking intermediate frequency may be twenty thousand and the guarding intermediate frequency may be fifty thousand. These two intermediate frequenciesmay be the same for all-of the carrier waves employed, as the non-interference between simultaneously communicatingpairs of radio stations is ensured by the non-interfering carrier wave lengths that are employed.

Briefly stated, the operation of the system is as follows: When an operator at one of the toll b oard positions has occasion to extend a connection through the medium of the radio or high frequency apparatus, she

inserts the plug of the connecting cord eircuit that is to be used into a multiple springjack of one of the trunks, the particular trunk to be used being indicated by the lighted condition of the assigning lamp associated with the spring-jack of that trunk. The insertion of the plug extinguishes the assigning lamps and lights the busy lamps associated with this trunk at all of the positions. It'also attaches the common register and sender apparatus to the trunk that has been taken for use, makes this trunk unavailable for reassignment until all of the trunks of the series have been assigned and used, and energizes the transmitting antenna system associated with this trunk to send out the calling carrier wave with its superposed guardin intermediate frequency. The attaching of tie common register and sender apparatus as a result of the insertion of the plug also sets into operation-apparatus associated with the common circuit that sue cessively tunes the receiving antenna system of the trunk to the calling station frequency the number of the distant radio or high frequency station with which connection is desired. These impulses are registered upon a station receiving register switch associated with the common register and sender apparatus. As soon as the channel testing switch and the station receiving register switch have received their registrations, a station transmitting register switch is automatically set into operation to cause impulses -of, the signaling intermediate frequency corresponding in number to the setting of the station receiving register switch to be sent out from the transmitting antenna system associated with the trunk that has been taken for use.

At each of the distant stations, one of the associated trunks is waiting in its preselected condition with its receiving antenna system energized and sensitive to the calling carrier wavewith its guarding and signaling intermediate frequencies. When the insertion of the plug at the calling station causes the sendingo-ut of the calling carrier wave with its guarding intermediate frequency, the pre selected trunk at each of the distant stations responds and automatically becomes connected with the associated common register and sender circuit, the assigning lamps associated with the preselected trunk at each distant station at the same time being extinguished to prevent these trunks from be ing used for calls originated at their respective stations. The response of the preselected trunk at each distant station also makes that trunk unavailable for reassignment until the remaining trunks of the associated series have each been used.

\Vhen the signaling intermediate frequency impulses controlled by the 'station transmitting register switch at the calling station are received at the distant stations, they cause the advance of the station select ing switches at the various distant stations to an extent corresponding to the setting of the station transmitting'register switch at the calling station. At the conclusion of the station selecting train of impulses only the apparatus at the particularstation that is being called remains responsive to further impulses from the calling station, the apparatus at all ofthe other stations being tion selectin train of impulses, the calling station apparatus operates to render the calling station trunk responsive to the receipt of the return signal.

If the particular station called for any reason has not been selected and the return signal is not received, a busy signal is automatically 7 given the calling station operator, the common sender and register apparatus is disconnected from the trunk, and the connection isreleased upon the subsequent withdrawal of the calling operators plug. iut'if the called station is properly selected and the return signal sent out, the calling station apparatus proceeds with the sending out of the next set of impulses. The number of impulses in this set or train is determined by the setting of the channel receiving register switch. and the impulse train causes a corresponding setting of the channel register switch at the distant selected station, the other distant stations being now irresponsive.

At the'end of the channel selecting train of impulses, the transmitting and receiving antennae systems of the calling and called stations are at once automatically tuned to complementary frequencies of the selected communicating pair, and the called station upon becoming tuned sends out a short return signal over its guarding intermediate frequency. It the tuning of the called station has not been properly effected, the failure'to receive this return signal at the calling station causes the common sender and register circuit at the calling station to become disconnected from the trunk, and a busy signal to be given the calling operator, who then releases the connection bv withdrawing the plug. If the return signal is received at the calling station, the talking conductors of the trunk are extended to the associated high frequency apparatus, and the common sender and register apparatus is released for assignment to another one of the associated trunks.

\Vhen the called station has been selected and the high frequency apparatus associated with its assigned or waiting trunk has been tuned to the selected channel or pair of communicating frequencies, a calling lamp associated with the answering spring jack of the trunk is lighted. This informs the operator of the call; and when the operator inserts a connecting circuit plug into the spring jack of the trunk thus indicated, the line lamp is extinguished, busy lamps associated with the trunk at the various toll board positions are lighted, and the Zsupervisory lamp of the calling operator'sconnecting circuit is extinguished.

The calling and called operato s may then extend the connection to the associated local lines that are to be connected. During the conversatiom each operator may signal the distant operator by causing the flashing of the distant supervisory lamp. At the end of the conversation, the withdrawal of the connecting circuit plug by the calling operator restores to normal the transmitting and receiving antennae tuning apparatus at both the calling and the called stations, renders inert the transmitting and receiving antennae systems of the calling station and the transmitting antenna system of the called station, and lights the called station operators supervisory lamp. The called station operator, upon disconnecting in response to the lighted supervisory lamp, releases the trunk that has been used and renders its receiving antenna system unresponsive.

Before proceeding with a detailed description of the operation of the system, we may briefly consider the normal or waiting condition of the trunks at the various stations as determined by the associated preselecting circuits. has been stated in connection with the description of Fig. 5, there are several trunks capable of use as inlets or outlets between the associated station and the other stations of the system. One of these trunks at each station is normally kept in condition to respond to a radio or high frequency call from any other station, and to be used for any outgoing call originated by one of the localoperators of the associated station; and this preselected trunk is also in condition to be come automatically attached to the common sender and register apparatus upon the receipt of an incoming call or the initiation of an outgoing call. This result is effected by means of a series preselecting circuit extending by way of conductors 12, 14 and 15 and through the contacts of relays 10, one of which relays is associated with each. of the trunks. In the initial condition of the preselecting circuit at each station all of the relays 10 are inert; and in this condition the preselected trunk is the one associated with the left-hand relay 10 of the series shown in Fig. 1.

When the first trunk is taken for use and is subsequently released from the common sender and register apparatus, the trunk associated with the second or middle relay 10 is automatically pre- At each of the stations, as

selected and placed in waiting condition; and when this trunk istaken for use and subsequently released from the common apparatus the third trunk, associated with the right-hand relay 10 of the series, is automatically preselected and place( in waiting condition. When the third trunk is taken for use and is released from the common apparatus, the relay 40 operates to release the relays 10 of such of the preceding trunks in the series as may in the meanwhile have become idle as a result of the termination of the communicating connections established over them, thus again placing these trunks in line for preselection or assignment. The preselected or assigned trunk at each station is connected with a normally closed energizing path includin resting contacts of the relays 51 and 52- o? the common sender and register apparatus, which closed path keeps the relay 16 of the associated trunk operated and the assigning lamps 31 associated with the spring jacks of this trunk lighted. The operated relay 16 holds closed a circuit by Way of conductor 240 that keeps the receivmg antenna system of the associated trunk energized and responsive to radio or high frequency waves that may be transmitted from the other stations of the system. The condition of preselection also places the multi-contact relay 11 of the preselectedtrunk under the control of the relays 17 and 20 of the trunk, so that if the relay 17 of this particular trunk is operated as a result of the initiation of a call at the local station, or if the relay 20 is operated as the result of the initiation of a call at one of the distant stations, the multi-contact relay 11 is actuated to connect the circuits of the preselected trunk with the circuits of the common sender and register apparatus. Thus, each of the stations normally has one of its trunks responsive to calls from other stations of the system and normally available for outgoing calls, and in condition to become attached automatically to the common sender and re ister apparatus upon being taken for use y either an incoming or outgoing call.

In detail, the operation of the system of the invention is as follows: When an operator at one of the positions of the toll board at one station wishes to establish connection with another station through the medium registering sleeve contacts of plug 36 and spring jack 33, and winding of relay 27 to ground. This causes the operation of relay 27 and the lighting of supervisory lamp (i, the su}')ervisory relay 38 at the time being inert. When relay 27'operates, it opens the energizing circuit of and extinguishes the assigning lamps 31 associated with this particular trunk at all of the toll board positions. At the same time, a circuit is closed that extends from battery through the winding of slow release relay 17 and right-hand front contact and armature of relay 27 to ground. Relay 1?, in operating. closes at its inner left-hand armature and front contact a circuit for causing the illumination of the busy lamps 32 associated with the assigned trunk at each of the toll board positions. The lighting of the lamps 32 serves to warn the operators at the various positions that connection has been established with this particular trunk. At its outer right hand armature and back contact, relay 1'7 breaks the circuit through the medium of which relay 18 is enabled to energize relay 20 upon the receipt of a call from a distant station, thus immediately rendering this particular trunk unavailable .for the receipt of incoming calls. At the same time, relay 17 brings about the energization of relay 2 to break this circuit at another point. At its outer right-hand armature and front contact, relay 17 applies aground to the conductor subsequently used for holding operated the busy test relay 24, the talking circuit closing relay 25, and the relay 10, of the associated trunk. The application of this ground also energizes relay 19 to operate relay 52. as will be described. At its inner right hand armature and front contact, relay 17 places in readiness the impulse circuit controlled by relay 27; and at its outer left hand armature and front contact relay 17 applies ground to a circuit that extends by way of the outer upper back contact and armature of the associated relay 10, assuming the pre-selccted trunk to be the third in the series. conductor 12, outer upper front contacts and armatures of the relays 10 of the other busy or unavailable trunks in series, conductor 13, outer lower armatures and front contacts of the relays 10 of the other busy or unavailable trunks in series by way of conductor 14, outer lower armature and back contact of relay 10 of the preselected trunk, and winding of relay 11 of that trunk to battery. This energizes relay 11 of the trunk, which thereupon closes its plurality of associated contacts, thus extending this particular trunk to the circuits of the common sender and register apparatus. \Vhen relay 11 operates it extends the ground that has caused its operation by way of the two lefthand armatures and front contacts of relay 11 to the winding of relay 9; and relay 9 in operating closes a circuit that extends from Ill battery through the lower winding of relay 10 of the preselected or assigned trunk and the front contact and armature of relay 9 to ground. The resultant operation of relay 10 breaks the initial energizing circuit completed by way of the outer upper and outer lower back contacts and armatures of the relays 10, as previously traced, and extends conductors 12 and 14 to the relay 10 of the next available trunk in the series. If the trunk whose relay 10- has thus been operated as described is the last available trunk of the series, the extension of the series circuit of conductors 12 and 14 causes the operation of the common relay 40 of the series circuit to release forreassigninent the trunks that have ceased to be busy, as will hereinafter be described. \Vhen relay 10 is operated by the operation of relay 17, as above described, it locks up by way of a circuit that extends from battery through the upper winding and inner upper front contact and armature of relay 10 and the back'contact and armature of slow release relay 40 to ground. At the same time, another holding circuit for relay 10 is completed from battery through the lower winding and inner lower front contact and armature of the relay, inner lower back contact and armature of relay 24, and outer right hand front contact and armature of relay 17 to ground.

The operation of relay 10, as has been stated, breaks the initial energizing circuit of the relays 11 and 9. But both of these relays are held energized by way of a circuit that extends through their windings and the left hand armatures and front contacts of relay 11, conductor 311, and lower front contact and armature of relay 52 of the common sender and register circuit to ground, relay 52 having been energized to attract its armatures the instant relay 1.1 operated, by way of a circuit that extends 'from battery through the winding of relay 52, conductor 305, contacts of relay 11, left hand armature and front contact of relay 19, middle lower back contact and armature of relay 20, outer lower back contact and armature of relay 24, and outer lower back contact and armature of relay 25 to ground. Relay 19 was operated at the moment the insertion of the plug 36 in the springjack 33 caused the operation of the relays 27 and 17 by way of a. circuit that extends from battery through the winding of relay 19, inner .lower back contact and armature of relay 24, and outer right hand front contact and armature of relay 17 to ground. Relay 19 also in operating closes at its right hand front contact and armature a branch to ground of the circuit that normally extends from battery through the w'nding of relay 241. windings of relays 242 and 243 in parallel, conductors 242' and 243, upper armature and back contact of relay 216 and right hand armature and back contact of relay- 215, conductor 240,. and front contact and armature of relay 16 to ground. This circuit, through the contacts of relay 19 is closed at the instant the trunk is taken for use by the operator, so that when relay 16 releases upon the operation of relay 10 and opens the nor-' mally closed energizing path for relays 241, 242 and 243 by way of the contacts of relay 16, these relays are maintained encrgized by the above-traced path closed at the right hand front contact and armature of relay 19. Another branch of the energ zing circuit of relays 241, 242 and 243 is closed by contacts of the relay 11 when that relay operates: so that these relays are held operated to keep the receiving antenna system of the preselected trunk in responsive condition, subject .to the control of relays 215, 216 and 221, from the moment the trunk is preselected for use until the plug 36 is removed from the springjack 33 at the endof the conversation, as will hereinafter be described.

\Vhen relay 52 of the common sender and register circuit operates upon the operation of the relay 11 of the preselectedtrunk, as has been described, it opens at its lower back contact and armature the path that subsequently will be used for the restoration of the switches of the sender and register circuit; and at its upper armature and back contact, it opens the path that extends by way of conductors 310 and 15 and the in'ddle upper armatures and front contacts of the relays 10 of the series circuit and the middle upper armature and back contact of the relay 10 of the first trunk of the series that is available for use to the assigning lamps 31 and the winding of the relay 16 of that trunk. As a result, although the operation of the relay 10 of the trunk that has just been taken for use extends the series circuit' to the apparatus of the first trunk that is available, the assigning lamps 31 ol. that trunk are not lighted, and the relay 16 is not energized to operate the associated rclays 241, 242 and 243, (Fig. 4) and render the trunk responsive, until the relay 52 of the common sender and register circuit is released after the common circuit has performed its functions.

The above described series of operations started bythe insertion of the plug 36 ir: the spring jack has caused the preselected trunk to become connected with the common sender and register circuit. The operator may now control the trunk apparatus through the medium of the sender and 125 register apparatus to call the desired high frequency or radio station, the apparatus at the same time proceeding automatically to select for use incommunicat-ing a pair of high frequency communicating waves or station, and then releases it.

' ing transmitted.

channels: that is. at the time idle an-l available. The operator controls the station selecting apparatus through the medium of the dial 39 associated with her connecting cord circuit 37. This dial is of the well known type'which operates to ell'ect a series of interruptions in the circuit that it controls, the number of interruptions in the series being determined by the degree of angular displacement of the dial by the operator. When the operator has connected the. calling dial with the cord circuit by operating the key 5, she moves the dial as far as is necessary to produce the required number of interruptions for selecting the desired Upon each interruption of the circuit at the dial contacts, the relay 2? releases its armatures. The duration of each interruption is too short to permit the slow release relay 17 to open the contacts which it controls. Consequently, ateaeh interruption of the circuit at the calling dial 39 a circuit is closed that extends from battery through the, winding of the stepping magnet a of station register switch A (Fig. 2) winding 01' the associated slow release relay 50, normal contact and wiper A of register switcii A. outer upper back cont-act and armature of relay 53, conductor 301, contacts of relay 11, inner right hand armature and iron contact of relay 17, and right hand back contact and armature of relay 27 to ground. The impulse thus produced causes the register switch A to take one step, thus breaking the initial energizing circuit by way of wiper A; but an alternate energizing path has been closed at the right hand front contact and armature of relay 5-0. and as the relav is slow in releasing, this energizing path through the winding of relay 50 and the winding of the stepping magnet 11 of switch A is maintained dui ng the impulses of the group that are now be-- The register switch A, therefore, takes one step for each of the impulses of the group, and at the end of the impulses line relay 27 remains energized, thus holding open the impulse circuit'by 'way of the winding of the relay 50 and the winding of the associated stepping magnet, 'thus permitting relay 50 to release. The advance of the switch wipers of the register switch A has caused the wiper A to rest upon a contact point corresponding in number to the number of impulses in the transmitted train; and the release of the relay 50 connect-s switch wiper A by wav of inner left hand back contact and armature of relay 50, outer lower back contact and-armature of relay 53, conductor 305, contacts of relay 11, left hand armature and front contact of relay 19,"middle lower back contact and armature of relay 20, enter lower back contact and armature of relay 2%, and outer lower back contact and armzr ture of relay 25 to ground. This applies ground to the contact upon which switch arm A is resting and identifies that contact. i

Concurrently with the registration of the number of-the desired toll station upon the register switch A, apparatus associated with the trunk has been operating to send out the common or calling carrier wave, and to select a pair of individual or communicating carrier waves for the subsequent establishment of communicatii'ig relation be.- tween the calling station and the called station. \Vhen relay 11 of the preselected trunk operates upon the insertion of plug 36 int-o springjack 30, it closes a circuit that extends from battery through the upper armature and back contact of relay 199 (Fig. 3), winding of relay 200, windings ot' relays 181, 191 and 201 in parallel, left hand armature and back contact of relay 114, conductor 34, contacts of relay 11, conductor 846, and inner upper armature and back contact of relay 53 to ground. Re lay 200, in operating, causes the energization of motor generator 146 to cause generators 14-7 and 148 to generate plate and filament current for the transmitting antenna system; and also causes the energization of relay 216 to render operative the guarding intermediate frequency oscillator of the transn'iitting antenna system. At the same time, relay 216 in operating opens at its upper armature and back contact the path by way of conductor 243 for energizing relay 243, thus making irresponsive the guarding intermediate frequency branch of the receiving antenna system. Relays 181, 191 and 201 in operating tune the transmitting antenna system to the frequency of the. calling carrier wave. Thus, the calling carrier wave with its guarding intermediate frequency is automatically transmitted from the calling station when the preselected trunk is taken for use. The ell'ect of this sending out of the calling Wave and its guarding intermediate frequency is to causethe response of the preselected trunk at each of the other toll stations of the system, and the connection of the common sender and register apparatus at each station with the associated prcselected'trunk in readiness to receive the impulses that are about to be transmitted from the calling station. Also the effect at each of the other stations of the system is to extinguish the assigning lamps 31 associated with the preselected .trunk, so that during the transmission of the impulses from the calling station the operators at-the other stations know by the absence of any lighted assigning lamp that no call may be initiated by them. The operationsby means of which these results are attained will be described in detail hereinafter.

Continuing now with the operations that are taking place at the calling station, when the common sender and register circuit is attached to the preselected trunk through the operation of therelay 11, a circuit is closed that extends from battery through the winding of relay 54 (Fig. 2), switch arm, B and its normal contact, aouter lower back contact and armature of relay 53, conductor 305, contacts of relay 11, left hard armature and front contact of relay 19, middle lower back contact and armature of relay 20, outer lower back contact and armature of relay 24, and outer lower back contact and armature of relay 25 to ground. Relay 54 in operating locks up by way of its inner left hand front contact and armature and inner contact of timer 59 to ground. Relay 54 in operating also closes a circuit that extends from battery through the winding of the stepping magnet 19 of switch B, the armature and back contact of this magnet, switch arm B and its normal contact, and outer right hand front contact and-armature of relay 54 to ground over' the circuit "previously traced by way of conductor 305. The effect of the closure of this circuit is to cause switchB to take one step, the advance of the switch arms tc their first position from normal breaking the energizing circuit of the stepping magnet at the switch B The above described movement of the switch .arms of switch B to their first position from normal closesa circuit that extends from battery through the winding of relay 221 (Fig. 4), winding of relay 322, conductor 165, contacts of relay 11, conductor 365, first contact from normal and switch arm B and outer right hand front contact and armature of relay 54 to ground by way of conductor 305. Relay 221 in operating opens the condenser branch 231 that normally tunes the receiving antenna circuit to the common carrier wave; and at the same time closes at its inner left hand armature and contact the energizing path for the re lay 243 which has heretofore been held open at the upper contacts-of the relay 216. Relay 322 in operating closes the condenser branch 332 that tunes the receiving antenna system to-one of the carrier wave frequen cies of the first individual pair. Relay 322 in operating also at its left hand front contact and armature connects its winding to the holding conductor 101 that is adapted at a later stage of the operation to be connected to ground at the outer lower front contact and armature of relay 25. As a result, therefore, of the first step taken by switch B on the operation of relay 54, the guarding intermediate frequency branch of the receiv- 'second position from normal.

mg antenna system is energized and the guarding intermediate frequency relay 252 (Fig. 4) is made responsive to the frequency of one of the waves of the first communicatin carrier wave pair or channel. As will su sequently be described, when any of the pairs of communicating carrier wave frequencies is in use between two of the radio stations, each of the two stations transmits a different one of the'high frequency waves of the pair with the talking or signaling and the guarding intermediate frequencies constantly impressed upon it. Therefore, if the first pair of individual carrier waves is in use when the receiving antenna system of the calling radio station is tuned to either one of the frequencies of this pair, (the tuning in the present instance being to the wave adapted to be transmitted by the calling station), the superposed guarding intermediate frequency is detected, amplified and recti lay 252 to ground. The stepping magnet b of switch B attracts its armature, and in doing so opens the short circuit that normally exists by way of the back contact of the stepping magnet armaturearound the high resistance winding of slow release relay 56. Relay 56 then operates and closes a circuit for the operation of relay 57 by way of front contact and armature of relay 56 and inner resting contact of timer 55 to ground. Relay 57 in operating locks up by way of its inner left hand front contact and armature, and inner contact of timer 55 to ground; and also, at its outer right hand front contact and armature, closes a circuit for the energization of timer 55. Timer 55 consequently starts the slow inward movement of its plunger, and in doing so opens the outer timer contact, which interrupts the ener izing circuit of the stepping magnet of swltch B; the circuit'of the stepping magnet, how- I even, having already been interrupted in effect by the breaking of the short circuit about the high resistance relay 56. The switch arms of switch B are thus caused to take another step, from their first to their In moving from its first to its second position, switch arm B breaks the energizing circuit that had previously existed for the operation of tuning relay 322 (Fig. 4), and closes, by way of conductors 166 and 366, an energlzing circuit for the operation of tuning relay 323. As the energizing path for the opof relay 323 changes the tuning of the re- "ceiving antenna system so that it becomes irresponsive to the calling station transmitting frequency of the first communicatlng carrier wave pair and becomes responsive to .the calling station transmitting frequency of .the second communicating carrier wave pair if the second pair of carrier waves 1s m use, the presence upon the carrier frequency of the superposed guarding intermediate frequency causing the energization of relay 252.

VVhen' the timer 55 completes its Inward movement. it opens its inner contact. 'lhis releases relay 57, which in turn deenergizes and permits the immediate return to nor- 4 "mal of timer 55. When the outer contacts are closed upon the return to normal of timer 55. if relay 252 has become energized as a. result of the secondpair of communicatingcarrier waves being in use, then a circuit is again completed for the energizatlon of the stepping magnet b of switch B by way of the armature and back contact of the magnet, outer contact of timer 55, outer left hand front contact and armature of relay 54, conductor 300, contacts of relay 11, conductor 100 and front contact and armature of relay 252 to ground. This causes another cycle of operations like that previously described. in the course of which switch B ad vances its wipers to their third contact position from normal, and a test is. made to determinethe available or unavailable condition of the third pair of communicating car rier waves. If this pair should also be found to be in use, and the relay 252 be consequently again energized. the switch 13 advances its wipers to their fourth contact position. As a result, a circuit is completed that extends from battery through the winding of relay 24 (Fig. "1), contacts of relay 11, conductor 307, fourth contact and wiper arm B of switch B, and outer right hand front contact and armature of relay 54 to ground bv way of conductor 305. Relay 24 in operating locks up by way of its inner lower front contact and armature, and outer right front contact and armatureof relay 17 to ground. At the same time, relay 24 in operating opens the talking circuit of the trunk, and closes a circuit that extends from battery by way of supervisory relay 38 of the connecting cord circuit, registering ring contacts of the plug 36 and spring-jack 33. inner upper armature and front contact of relay 24, source of busy tone current 45, and busy back interrupter 41 to ground. The closure of this circuit operates superviso y y 38 t9 a e t e int rm tentflashingof the cord circuit supervisory lamp 6, and to produce a bus .tone in any telephone receiver that is at t e time associated with the connecting cord circuit..-

Upon seeing the flashing of the lamp or hearing the busy tone, the operator may withdraw the plug 36 from the spring jac 33, whereupon the apparatus of the trunk that has been taken for use is restored to normal in a manner that will subsequentlybe described. Also the breaking of the outer lower and inner lower resting contacts of the relay 24 interrupts the circuit that has been maintaining the relay 52 of the common sender and register circuit operated, and that relay in releasing causes the release and restoration of the common sender and register to one of the first pair of communicating carrier; waves, the guarding intermediate frequency relay 252 remains inert. Therefore, no circuit is closed by way of the contacts of this relay for causing the switch B to take another step, and for causing the energization of the relay 56 and consequent energization of the relay 57. At the moment that the relay 54 was first energized, a circuit was closed extending from battery through the winding of relay 58, outer contact of timer 59, inner right front contact and armature of relay 54, and outer right back contact and armature of relay 57 to ground. The operation of relay 58 closes the energizing circuit of the timer 59. and the timer starts the slow inward movement of its plunger. thus breaking the outer resting contact of the timer includedin the initial energizing circuit of relay 58. But relay 58 remains energized by way of the locking circuit through its right front contact and armature. As long as the energizing circuit of relay 58 remains closed the inward movement of the plunger of timer 59. continues: and if the relay 57 remains unenergized for a period of time determined by the setting of the timer 59, the timer completes its inward movement and opens its inner contacts, thus releasing relay 54. The interval of time required for the tuning of the receiving antenna system to each pair of the various communicating carrier waves, and for the response to the presence of the tested wave, is less than that required for the timer 59 to complete the inward movement of its plungerzso that each pair of communicatin use causes the operation of relay 57 and the release of relay 58, and consequently of the timer 59, before the timer has moved its plunger far enough toope'n its inner contacts and release relay 54.

Underthc present assumption, that the first pair of communicating carrier Waves or channel is found available, is permitted to complete'its movement and release relay 54, and that relay in releasing opens, at its outer right hand front contact and armature, the energizing circuit of the receiving antenna tuning branch of the wave frequency under test, thus reestablishing the normal tuning of the receiving system, and at the same timecloses a circuit for starting the operation of the impulse mechanism 600 (Fig- 2) to send out station selecting impulses in accordance with the setting of register switch A, and carrier wave or channel impulses in accordance with the setting of register switch B.

Before proceeding with a description of the further operations at the calling station, we may turn to a consideration of the effects produced at the distant stations by the initiation of the call. The normal condition at the various radio stations is one in which one or another of the trunks of each station has been preselected for use, and is standing in readiness to respond to the calling carrier wave with its intermediate guarding frequoncy sent out when any other station calls. The preselection or assignment of a trunk causes the energiza: tion of the'relay 16 and the lighting of assigning lamps 31 of that trunk over a circuit closed by way of conductor 15 when relays 51 and 52 fall back upon the release of the common sender and register circuit and the relay 16 in operating closes a circuit that extends from battery through the windings of the relays 241, 242 and 243, (Fig. 4), conductors 242 and 243, resting contacts of relay 215 and 216, conductor 240, and front contact and armature of relay 16 to ground. Therefore, relays 241, 242 and 243 of the preselected trunk at each station are held energized to cause the application of filament current to the thermionic tubes of the receiving antenna system, and thus keep thecorrespending trunk in a responsive condition.

Upon the sending out from the calling station of the calling carrier wave with its guarding intermediate frequency, 'as has been described, the relay 252 (Fig. 4) associated with theupreselected trunk at each station responds, and in responding closes a circuit that extends from battery through the winding of relay 18 (Fig. 1), conductor 100, and front contact and armature of relay 252 to ground. hen relay 18 operates as a result of the closure of th s circuit, it opens at its outer left back contact and artact relay 11 of the preselected trunk, outer mature the energizing c rcuit that normally keeps lighted the assigning lamps 31 associated with the preselected trunk at the various positions of the toll board. The extinguishing of the assigning lamps 31 indicates to the operators at the various POSi-r tions that for the time being there is no trunk available for outgoing connections. The operation of relay 18 also closes a circuit that extends from battery through the wind ng of relay 20, inner left front contact and armature of relay 18, right back contact and armature of relay 2, and outer right back contact and armature of relay 17 to ground; and relay 20 locks up by way of its inner upper front contact and armature and the inner contact of timer 21 to ground.

When relay 20 operates it applies at its outer upper front contact and armature a ground to the holding conductor; and in doing so energizes relay 19. Relay 19, in operating, closes a branch to ground from conductor 240 for keeping energized the relays 241, 242 and 243 (Fig. 4) that maintain the energization of the thermionic tubes of the receiving antenna system. Relay 20 in operating closes a circuit that extends from battery through the winding of the multi-conlower back contact and armature of relay 10 of the preselected trunk, thence through conductors 14 and 12 and the closed outer lower and outer upper' fron t contacts and armatures of the relays 10 of the preceding trunks in the series to the outer upper armature and back contact of relay 10 of the preselected trunk, and thence by way of the inner lower front contact and armature of relay 2-0 to ground. This causes the relay 11 of the preselected trunk to operate and close all of its contacts to attach the common sender and register apparatus. Relay 11 in operating also closes at its left hand armatures and contacts a branch circuit from the operating path for energizing relay 9, and that relay in operating closes an energizing circuit including the lower winding of the associated relay 1(). Relay 10, in operating, locks up by way of its upper winding, its inner upper front contact and armature, and 115 the back contact and armature of the c0mmou relay 40; and also opens at its outerlower armature and back contact the initial energiz ng circuit of the relays l1 and 9, these relays thereafter being held energized by a circuit including conductor 2511 and the lowerfront contact and armature of relay 52, as willpresently be described.

When relay 20 operates, it also closes a circuit that extends from battery through the winding of relay 53 (Fig. 2), conductor 809, closed contacts of relay 11 of the preselected trunk, middle lower front contact and armature of relay 20, outer lower back contact and armature of relay 24, and outer 

